In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility as polysilicon or microcrystalline silicon and having uniform element characteristics as amorphous silicon. A metal oxide is used for various applications. For example, indium oxide which is a well-known metal oxide is used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors each of which includes a channel formation region formed using such a metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2).